01 - Set oven temperature to 375°F.
02 - Boil jumbo pasta shells in salted water until al dente. Drain and arrange on a baking sheet to cool.
03 - In a large bowl, combine ricotta, mozzarella, Parmesan, egg, spinach, basil, garlic, salt, and pepper. Mix thoroughly.
04 - Spread 1 cup of marinara sauce evenly in a 9x13-inch baking dish.
05 - Fill each pasta shell with approximately 2 tablespoons of the cheese and spinach mixture. Place shells in the baking dish.
06 - Pour remaining marinara sauce over stuffed shells. Sprinkle with remaining mozzarella and Parmesan cheeses.
07 - Cover dish with foil and bake for 25 minutes.
08 - Remove foil and bake an additional 10 minutes, until cheese is bubbly and golden.
09 - Allow dish to rest for 5 minutes before plating.